San Francisco recently instituted a citywide ban on plastic bags, then started charging a ten-cent fee for the paper bags you have to use instead. The idea is to promote reusable bags, but the law is also designed to rid the city of plastic litter—this would benefit the environment, because paper bags are considered more ecologically friendly. Right? Not quite.

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The notion that paper bags are more environmentally friendly than plastic is a "popular misconception," according to a 2005 report for the Scottish government. "It depends on what environmental issues you see as being more important," Lisa Mastny, director of the Worldwatch Institute's consumption project, told the WSJ. As it turns out, paper and plastic bags both carry significant environmental costs. But the costs occur on different ends of their operational life cycles. In the eternal dilemma of paper versus plastic, there just isn't an easy answer.

Americans have been toting groceries in brown paper since 1852, when Francis Wolle patented his automated paper-bag-making machine. Then, as now, it takes a lot of resources to actually make a tree into a sheet of paper. First, you've got to cut down large swaths of trees—14,000,000 trees annually in order to produce the 10,000,000,000 bags used in the US alone—using heavy-duty gas-fed machinery. Then you have to store the cut timber for three years as it dries, then ship it to a saw mill using trucks that run on diesel. There, more heavy machinery de-barks, chips, and then cooks the logs into pulp with the help of limestone and acid. It is then spread onto a wire mesh to dry and be rolled into paper.

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This process is laughably inefficient—producing a single ton of paper pulp requires three tons of wood and thousands of gallons of fresh water, in a 400:1 ratio of water to pulp. The paper bags themselves are heavy. Shipping a load of them, compared to plastic bags, can require seven times as many trucks, with all their gas and emissions. (2,000 plastic bags, for example, weigh about 30 pounds. 2,000 paper bags—280 pounds.)

It starts off well enough—most industrial timber in the US starts on a sustainable, managed tree farms. But in the end, paper products are the single largest contributor to America's municipal solid waste (MSW), constituting 29 percent of the country's annual garbage stream and half of all landfilled material.

On the other hand, paper bags are more readily recycled, reused—even composted—than their plastic counterparts. Recycling simply involves washing the bags in hydrogen peroxide, sodium silicate and sodium hydroxide to bleach and separate the fibers back into pulp for re-rolling. In addition, nearly 80 percent of Americans have access to recycling programs (either curbside or drop off). In 2010, nearly 63 percent of them participated, making paper the most recycled material as well. Paper bags will also decompose within a month when buried in an aerobic environment. They biodegrade within centuries when buried in anaerobic environments like landfills.

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Plastic bags, on the other hand, are far cheaper to make and transport, but they're incredibly difficult to recycle. Plastic is composed of numerous elements—carbon, hydrogen, oxygen, nitrogen, chlorine, and sulfur—derived from oil and melded into long chemical chains known as polymers. Plastic shopping bags were an instant hit with consumers and grocers alike when they debuted in 1977. The industry has grown to account for roughly four percent of the total worldwide oil use annually.

Now, plastic come in five basic types of polymers. The bags are typically made from low-density polyethylene (LDPE aka #4 plastic). Milk jugs and reusable water bottles are made from high-density polyethylene (HDPE aka #2 plastic) or polyethylene terephthalate (PET aka #1 plastic). Production of either requires 94 percent less water and 40 percent less power than paper bags do. In total terms of materials and energy, 1.75 kilograms of oil is required to produce a kilogram of #2 plastic. The significant weight and space savings reduce transportation requirements and cut the resulting air pollution in half.

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That's not to say the production process is green by any stretch of the imagination—five of the six most hazardous chemicals to work with, as ranked by the EPA, are commonly used in plastic production.

The larger problem is what to do with plastic bags once they've served their purpose. Americans threw away 31 million tons of plastic in 2010, 12.4 percent of the total MSW, constituting 11 percent of landfill waste. Sure, 90 percent of Americans report that they reused their plastic bags at least once (just think of all the dog poop picked up. But they actually recycle only eight percent of them—a measly 2.4 million tons in total.

The number is so low because plastic bags don't really lend themselves to recycling in the first place. The process generally involves sorting the plastic by type (usually by hand), shredding, and then melting it down to be reformed. This is an expensive procedure, averaging $4,000 per ton recycled. Unfortunately, when the polymer chains are broken, they don't readily recombine, which results in a lower quality plastic than what you started with. In the early days of composite decking, you could actually see the scraps of old grocery bags.

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Recycling also has its own shipping footprint. The Washington Post also reports that much of the plastic destined for "recycling" is actually shunted over to China and India, where it is incinerated thanks to the countries' lax environmental standards. Don't bother just throwing plastic bags away, either. When stored in anaerobic landfills, plastic bags can require thousands of years to decompose, if they ever do at all.

So the question we should be asking isn't which is better for the environment. It's more like: Which will screw the environment less in the long run? The answer is neither. If you really want to be green, bring your own reusable cloth bags the next time you head to the store.